My boy is really sick. He's been lethargic with clamped fins and is panting. He was fine until I did a water change a week ago. I did nothing different with his water change. He always hates them, but is fine the next day. I use Prime and stress coat and I always let the water sit.

He doesn't have any ich spots, but he is losing his red color and an orange color on his body.

Housing
What size is your tank? 5 gal marine land hex
What temperature is your tank? 78
Does your tank have a filter? Yes - baffled
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Aqueon betta pellets or API pellets. Frozen or freeze dried daphnia and freeze dried bloodworms. A pea on Sunday
How often do you feed your betta fish? 2 times a day - 2-4 pellets determined by size.

Maintenance
How often do you perform a water change? Weekly
What percentage of the water do you change when you perform a water change?
Weekly 3 50% changes then a 100% change.
What type of additives do you add to the water when you perform a water change? Prime and stress coat

Water Parameters:
Have you tested your water? If so, what are the following parameters? Yes

Ammonia: 0
Nitrite:0
Nitrate:0
pH:7.5
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? Paler with a missing scale
How has your betta fish's behavior changed? Lethargic and clamped
When did you start noticing the symptoms? 3 days ago
Have you started treating your fish? If so, how? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? 2.5 yrs

I would make a 50% water only change and double dose the dechlorinator-turn off the light for now. If he is struggling to swim-lower the water level.

To understand-he is in a 5gal filtered tank and you make 3 50% and 1-100% every week...If so, I would reduce your water changes to 50% weekly with vacuum-since you have a filter. Any live plants?

How soon after the water changes did he start to have problems and was this a 100% water change and/or did you cup him for the water change-If so, how did you re-acclimate.

If you did everything right-proper acclimation, used like temp dechlorinated replacement water...etc.....He could be aging out and nothing you did or didn't do-it just so happened to happen after a water change.....

Anytime you cup or remove the Betta from the tank for water changes-especially 100%-Its a good idea to acclimate like you would with a new fish. By adding small amounts of the new water in the holding container over at least 10-15min or to tolerance-then add the Betta back to the tank. A lot of different things can change in the chemistry with both the source water and tank water-usually it isn't enough to cause a problem-but sometimes it can and best to error on the side of caution-Then you have risk of forgotten dechlorinator or extreme temp changes...Not saying that is what happened in your case.

As a general rule-anytime a fish has a behavior change after a water change-make another water only change- leaving them in the tank- using double dose like temp dechlorinated replacement water-this is to rule out the water/environment as the cause first. Then double check the water temp with an independent thermometer. Next, check the heater itself for cracks-especially if you didn't let it cool off long enough or forgot to unplug it-a stray volt can cause behavior changes too.